1. Field of the Invention:
This invention relates to a switching circuit of an output from an amplifier, and more specifically to the switching circuit of an output from an amplifier of which output gain is controlled by a negative feedback.
2. Description of the Related Art:
An amplifier having a negative feedback has an output gain controlled by the negative feedback. However, when the output controlled by the negative feedback is applied to a load through a switch, the output voltage obtained across the load decreases by a voltage drop across the switch. Especially, in a semiconductor integrated circuit, the switch is formed by one or more transistors. One example of the switch is a transfer gate in which P-and N-channel MOS field effect transistors are connected in parallel with each other. Such transfer gate or other transistor switch has 10 to 100 ohms of ON-resistance. The ON-resistance causes the voltage drop across the switch. The voltage drop across the switch is serious when the load has a small impedance comparative to the ON-resistance.
The ON-resistance of the transfer gate or other transistor switch may be decreased by increasing the size of the transistor used therein. This measure, however, accompanies an increment of chip area of the semiconductor integrated circuit, resulting in rising a production cost, decreasing a production yield, lowering an integration density and other drawbacks.